The rules
- Always follow the packet first.
- The RHS says to cover seed with sieved compost, usually no more than the depth of the seed itself.
- Clemson Extension suggests a depth of two times the seed's diameter, then firming the mix.
- Very fine seed is scattered on the surface and left uncovered, or barely dusted.
- Large seeds (beans, peas, squash, sweetcorn) go deeper, as the seed contains more reserve and also dries out less.
Dry soil and light soil
Quick-draining, sandy soil dries out near the surface, so seed can go slightly deeper than in heavy, moisture-holding soil. Sowing in wet, cold soil outdoors invites rot, which is why it pays to wait for the soil to warm.
Depth calculator
How deep for this seed?
Enter the width of a seed.
Width of the seed at its thickest. A rule of thumb for larger seed; the packet comes first.
Outdoors
The same rules apply in the garden. See sowing seeds outdoors.